1. Field of the Invention
The present invention relates to a method of producing an electrode for a battery and the electrode produced by the method. More particularly, the present invention relates to a method of forming a metallic porous foil from metal powder, filling a void of the metallic porous foil with a powder of an active substance, and fixing it to a surface of a sheet of the metallic porous foil. The method is preferably used to produce a negative electrode of a nickel hydrogen battery. In addition, the method is preferably used to produce a positive electrode of the nickel hydrogen battery, and further to produce electrodes of various kinds of batteries such as a nickel cadmium battery, a lithium primary battery, a lithium secondary battery, an alkaline dry cell, a fuel cell, and a battery for a vehicle.
2. Description of the Related Art
Conventionally, a negative electrode of the nickel hydrogen battery is produced as follows: Hydrogen-storing alloy powders are kneaded with binder (binding agent), carbon (electrically conductive material), and the like to obtain a pasty active substance. Then, the pasty active substance is filled into three-dimensional pores of a three-dimensional metallic porous plate such as a foamed sheet-shaped metallic porous plate and a nonwoven fabric-like metallic porous plate used as a base material of an electrode; or the pasty active substance is applied to a metallic porous plate produced by forming pores on a metallic plate such as a punching metal, a lath, and the like. Finally, the metallic porous plate is passed between a pair of pressure rollers one to four times to pressurize the plate after the pasty active substance is dried to produce the electrode.
However when the metallic porus plate having the three-dimensional pores is pressurized after the pasty active substance is applied thereto, a skeleton surrounding the three-dimensional pores is destroyed by the powder of the active substance. Therefore, an electrode thus formed is not flexible and is hard.
More specifically, the thickness of the skeleton surrounding the pores of the foamed metallic porous plate and the nonwoven cloth-like metallic porous plate is as small as 30-50 xcexcm, and the hydrogen-storing alloy powders used as the active substance of the nickel hydrogen battery are hard. Thus, the hydrogen-storing alloy powders may destroy the skeleton of the pores of the foamed metallic porous plate and the nonwoven cloth-like metallic porous plate.
On the other hand, the metallic porous plate produced by forming pores on the metallic plate such as the punching metal, the lath, and the like has a high degree of strength. Thus, the metallic porous plate is not destroyed by the hydrogen-storing alloy powders. The pores formed on the metallic plate are not three-dimensional. Therefore, to fix the hydrogen-storing alloy powders to the metallic porous plate, it is necessary to apply the pasty hydrogen-storing alloy powders thereto and pressurize the metallic porous plate repeatedly at a high degree after the pasty hydrogen-storing alloy powders are dried. However, when the metallic porous plate is pressurized repeatedly at a high degree, an electrode thus produced is very hard.
In the case where the negative electrode having base material, in which the hydrogen-storing alloy powders have been filled, is used for a cylindrical battery, the negative electrode is spirally wound together with a positive electrode via a separator to accommodate both electrodes in a battery can.
However, because the negative electrode produced as described above is hard, the electrode is cracked when it is wound. Normally, the electrode is accommodated in a battery can, with a crack left thereon. The occurrence of the crack causes an alloy powder layer of the active substance to drop from an electricity collecting material. As a result, flow of electric current is not favorable in the electrode and the electric resistance becomes high, which deteriorates the characteristic of the battery. Therefore, in a conventional method, preventive measures are taken by forming fine cracks intentionally in the electrode to prevent large cracks from being generated when it is wound. However, the alloy powder layer drops from the cracks. Thus, the preventive measures cannot solve the above-described problem that causes the characteristics of the battery to deteriorate.
Further, according to the conventional method, a pasty binder is kneaded with powder of hydrogen-storing alloy powder, and a mixture thereof is applied to a metallic porous plate. According to this method, the entire surface of alloy powders is likely to be coated with the binder. In this case, the alloy powders do not contact each other, and the binder disturbs the flow of electric current. Especially, electricity collection performance at a thickness of the electrode decreases, which deteriorates a characteristic of the battery.
The present invention has been made in view of the above-described situation. Thus, it is a first object of the present invention to provide an electrode for a battery which does not become hard when pressurization is repeated to securely fill an active substance such as hydrogen-storing alloy powder into a base material of an electrode and fix it thereto and which does not crack when the electrode is wound spirally.
It is a second object of the present invention to improve electricity collection performance by contacting powders of the active substance with each other directly.
In order to achieve the object, there is provided a method of producing an electrode for a battery comprising the continuous steps of:
forming a metallic porous foil consisting of metal powders in which adjacent powders are contacted and bonded with each other and gaps between non-contact powders form fine voids;
applying powders of an active substance not containing a binder to a surface of the metallic porous foil at a required position while the metallic porous foil is being conveyed continuously;
filling the powders of the active substance into the fine voids of the metallic porous foil and fixing the powders on the surface of the metallic porous foil under pressure by passing the metallic porous foil between a pair of rollers immediately after the powders of the active substance is applied to the metallic porous foil or while the powders of the active substance is being applied to the metallic porous foil;
forming a binder coating layer on surfaces of the powders of the active substance positioned at the surface of the metallic porous foil by introducing the metallic porous foil into a tank accommodating a liquid binder;
drying the binder coating layer by introducing the metallic porous foil into a drying oven;
and setting the metallic porous foil to a required thickness by passing the metallic porous foil sequentially between a plurality of a pair of pressure rollers arranged along a conveying path.
For example, the powders of the active substance not containing a binder are applied to both surfaces of the metallic porous foil by introducing the metallic porous foil into a hopper accommodating the powders of the active substance. The metallic porous foil, which has the powders of the active substance applied to it, is passed under pressure between a pair of rollers disposed at an exit position of the hopper so that the powders of the active substance are filled into the fine voids of the metallic porous foil and fixed to both surfaces of the metallic porous foil. Thereafter, the metallic porous foil is introduced into a tank accommodating a liquid binder so that binder coating layers are formed on the surfaces of the powders of the active substance, which have been applied to the both surfaces of the metallic porous foil.
According to the present invention, as described above, the metallic porous foil consisting of metal powders and having fine voids consisting of gaps between non-contact portions of the metal powders is used as the base material of the electrode. Because the metallic porous foil has the fine voids, powders of the active substance can be filled into the voids.
The metallic porous foil is entirely flexible, and the gaps between adjacent powders can be increased and decreased, whereby the powders of the active substance are filled into the fine voids. Thus, when the metallic porous foil is passed between a plurality of pressure rollers after the powders of the active substance have been applied to the metallic porous foil, the powders of the active substance penetrate gradually into the voids consisting of the gaps between metal powders of the metallic porous foil. In other words, the metallic porous foil penetrates into the gaps between the adjacent powders of the active substance while the metallic porous foil is deflecting. In this state, the metallic porous foil serves as a cushioning medium between the adjacent powders of the active substance. Therefore, even though the metallic porous foil is pressurized repeatedly, it does not become hard and holds its flexibility, unlike the conventional metallic porous foil, and further the holding force of the alloy powders of the active substance by the metallic porous foil is increased.
As described above, when the metallic porous foil is pressurized repeatedly, the metallic porous foil penetrates into the gaps between the powders of the active substance while the metallic porous foil is deflecting. In other words, the powders of the active substance deflect the metallic porous foil. Accordingly, it is possible to produce an electrode is not hard, but is flexible. Therefore, when the electrode is used for a cylindrical battery, the electrode can be wound easily without cracking of the electrode. Further, the base material consisting of the metallic porous foil penetrates into the gaps between the adjacent powders of the active substance and strongly holds the powders of the active substance. Thus, it is possible to prevent a layer of the active substance from dropping from the base material.
Further, because the powders of the active substance that are applied to the metallic porous foil composing the base material of the electrode do not contain a binder, the powders of the active substance contact each other directly. Therefore, it is possible to solve the conventional problem of deterioration of the electricity collection performance of the metallic porous foil, which occurs as a result of the non-contact of the powders of the binder-containing active substance caused by the presence of the binder between the adjacent powders of the active substance applied to the metallic porous foil. Thus, it is possible to improve battery characteristics of cylindrical and square pillar-shaped batteries.
As described above, the powders of the active substance not containing the binder are applied to the metallic porous foil, then the metallic porous foil is pressurized by a roller to fill the powders of the active substance into voids thereof and to fix them to the surface thereof, and then the metallic porous foil is introduced into a liquid binder tank to immerse the metallic porous foil in the binder. In this process, the binder forms a thin coating layer on the surface of the layer of the powders of the active substance. Thus, it is possible to prevent the powders of the active substance from dropping from the layer thereof formed on the surface of the metallic porous foil. Moreover, the binder penetrates into gaps between non-contact portions of the adjacent powders of the active substance and into remaining gaps between the powders of the active substance and the metallic porous foil. Thus, the binder adheres the powders of the active substance strongly thereto without preventing direct contact of the powders of the active substance and fixes the powders of the active substance and the metallic porous foil strongly to each other. That is, the binder can be used for only the fixing action without preventing flow of electric current.
Further, in the present invention, subsequent to the process of forming the metallic porous foil composing the base material for an electrode from metal powders, the powders of the active substance are continuously supplied to the metallic porous foil at a required pressure. In this manner, an electrode can be manufactured continuously. Accordingly, it is possible to improve the productivity of the electrode and thus produce it at a low cost.
The metallic porous foil is formed from nickel powder, and the powders of the active substance are composed mainly of hydrogen-storing alloy powder. The metallic porous foil of the nickel powder is preferably used in producing a negative electrode of a nickel hydrogen battery. In the case of the negative electrode of the nickel hydrogen battery, preferably the hydrogen-storing alloy powder to be used as the powder of the active substance has an average diameter of 10 xcexcm-100 xcexcm. Preferably, the density of the hydrogen-storing alloy powder is 5.0-6.5 g/cc, when it is filled into and fixed to the electrode.
The powder of the active substance is not limited to the hydrogen-storing alloy powder, but the hydrogen-storing alloy powder or a mixture of the hydrogen-storing alloy powder and Ni powder or/and a transition metal powder such as Cu powder or the like may be used to produce the negative electrode of the nickel hydrogen battery. As described above, electricity collection performance thereof can be enhanced by mixing the hydrogen-storing alloy powder with the Ni powder, instead of carbon hitherto used as an electrically conductive material.
As described above, the powder of the active substance is composed of metal powder or alloy powder without adding a binder, the surface of each metal powder and that of each alloy powder is not coated with the binder, and the powders contact each other directly. Thus, electric current flows through the active substance preferably and electric resistance can be reduced. Thus, battery characteristics can be improved.
The powder of the active substance is pressed against the metallic porous foil with a roller, thereafter a powder of a transition metal is applied on the metallic porous foil on the surface of a layer of the active substance powder, and then the metallic porous foil is pressed with a roller. Continuously, the metallic porous foil is introduced into a liquid binder tank. According to the above steps, the surface of the layer of the hydrogen-storing alloy powder is coated with a layer of the transition metal, and the surface of the layer of the transition metal is coated with a binder layer. It is also possible for the liquid binder to contain the powder of the transition metal to form a binder coating layer containing the transition metal.
That is, in the case where it is preferable to coat the surface of the active substance layer with the transition metal, the transition metal is applied to the surface of the layer of the hydrogen-storing alloy powder in a later process, separately from the hydrogen-storing alloy powder.
The metallic porous foil may be produced as follows:
The metal powder is spread on a conveyor belt or a supporting sheet placed on the conveyor belt which is conveying continuously. Then, the conveyor belt or the supporting sheet on which the metal powder has been spread is passed between a pair of rollers to press the metal powder at a low force. As a result, adjacent metal powders contact partly each other, with gaps present between them. Then, the metal powder on the conveyor belt or on the supporting sheet is introduced into a sintering oven to sinter the metal powder. Then, the metal powder is separated from the conveyor belt or the supporting sheet.
As the conveyor belt, a solid metal sheet, an inorganic sheet including a metallic porous sheet or a layer of these sheets is used in a circulation driving apparatus of belt conveyor type. For example, the conveyor belt is made of SUS (310S). Metal powder spread on the conveyor belt is rolled at a low pressure and sintered to form a sheet, and the sheet can be separated from the surface of the conveyor belt. By introducing the conveyor belt that is moved continuously into the sintering oven, it is possible to form the metallic porous foil from metal powder continuously and very efficiently.
As described above, when the metal powders spread on the conveyor belt are rolled at a low pressure, spherical surfaces of adjacent metal powders are in point contact or line contact. That is, they do not contact each other entirely but have gaps present between them. Thus, when they are heated in the above-described contact state in the sintering oven, portions thereof in contact with each other are fixed with each other to form the metallic porous foil continuously, with gaps between the metal powders forming fine voids. The size of the formed void of the metallic porous foil depends on the size of the metal powder. That is, when the diameter of the metal powder is large, the void is large, whereas when the diameter thereof is small, the void is small. The metal powder having a diameter of 0.1 xcexcm-100 xcexcm is preferably used.
The kinds of metal for the metallic porous foil is not limited to a specific one, but the following metals can be preferably used: Ni, Cu, Al, Ag, Fe, Zn, In, Ti, Pb, V, Cr, Co, Sn, Au, Sb, C, Ca, Mo, P, W, Rh, Mn, B, Si, Ge, Se, La, Ga, and Ir; oxides and sulfides of these metals; and a substance or a mixture containing a compound of these metals. That is, it is possible to use Al, Ti, and V that cannot be used in electroplating. It is also possible to use powders of these metals alone or by mixing powders of a plurality of these metals with each other. It is preferable that the powders of these metals do not interlock with each other and are dispersive. Therefore, it is preferable that peripheral surfaces of these metals do not have concave and convex portions that interlock with each other. For example, preferably, these metals are spherical, die-shaped, square pillar-shaped, columnar or the like.
When the conveyor belt is porous, metal powder spread thereon drops from the pores of the conveyor belt. Thus, portions corresponding to the pores form through-voids of a metallic porous foil. The through-voids are larger than fine gaps between adjacent metal; powders. Thus, the metallic porous foil produced has fine voids and comparatively large through-voids.
According to the present invention, the metallic porous foil may be formed as follows: The supporting sheet is conveyed continuously; metal powders are spread on the supporting sheet; the supporting sheet on which the metal powders have been spread is conveyed over the conveyor belt; and the supporting sheet and the conveyor belt are passed between a pair of pressure rollers to roll them at a low pressure, with gaps between adjacent metal powders kept. Then they are introduced into a sintering oven to sinter the metal powders, whereby contact portions of the metal powders are fixed with each other and the fine gaps form fine voids. The following sheets can be used preferably as the supporting sheet: an organic sheet including a solid resinous sheet, a three-dimensional reticulate resinous sheet, a porous fibrous resinous sheet; and an inorganic sheet including a solid metal sheet, a metallic porous sheet or a laminate of these sheets.
The metallic porous foil formed by using the supporting sheet can be separated from the conveyor belt more easily than the one formed by spreading the metal powder directly on the conveyor belt of the circulation driving apparatus. Of the a above-described supporting sheets, the resinous sheet is burnt in the resin removal oven. On the other hand, the inorganic sheet such as the metal sheet is not removed by heating. In some cases, the inorganic sheet is separated from the formed metallic porous foil when it is discharged from the sintering oven. In the other case, it is conveyed downstream together with the formed metallic porous foil and wound together therewith. As a result of using a thin metal sheet as the supporting sheet, it is possible to increase the conveying speed and enhance productivity.
In the case where a porous sheet having a large number of pores is used as the supporting sheet, similarly to the case where the conveyor belt is used, it is possible to produce a metallic porous foil having fine voids consisting of gaps present in the adjacent metal powders and large through-pores in the portion thereof corresponding to the pores formed on the supporting sheet.
The conveyor belt or the supporting sheet on which the metal powder has been spread is introduced into a cooling oven disposed continuously with the sintering oven to cool the metal powder after it is sintered.
It is possible to use an electrode substrate, consisting of the metallic porous foil formed by merely sintering the metal powder spread on the conveyor belt or the supporting sheet in the sintering oven without passing it between a pair of the pressure rollers. However, there is a case in which a small number of metal powders contact each other and thus a desired degree of strength cannot be obtained. Therefore, it is preferable to increase the number of connection portions of metal powders by rolling them at a low pressure before they are introduced into the sintering oven after they are spread over the conveyor belt or the supporting sheet.
A mixture of a sublimable fine fragment, which can be burnt, and metal powder may be spread on a conveyor belt or the supporting sheet. Otherwise, the sublimable fine fragment may be spread thereon before the metal powder is spread thereon. Then, the sublimable fine fragment is burnt in a resin removal oven. In this manner, it is possible to produce a metallic porous foil having fine voids consisting of gaps between the adjacent metal powders and voids formed at portions where the sublimable fine fragment is burnt. In the case where a foamed agent or the like, which is decomposed by heating and generates gas, is used as the sublimable fine fragment. through-voids are obtained by the generated gas. In this manner, it is possible to produce a metallic porous foil having through-pores and further, to control the size of the through-void, depending on the size of particles of the sublimable fine fragment.
According to a method other than the above-described methods, the metallic porous foil is formed continuously as follows: That is, metal powders are spread directly over the surface of a pair of pressure rollers. Then, the metal powders are pressed by the pressure rollers at a required force to connect contact portions of the adjacent metal powders with each other and form gaps of non-contact portions thereof as fine voids. It is possible to use a pattern roller as a pair of pressure rollers disclosed in Japanese Patent Application Laid-Open No. 9-287006. The pattern roller has a pattern of a large number of concave portions formed on its peripheral surface to successively form a metallic porous foil having fine voids consisting of gaps between adjacent metal powders and large voids formed by using the pattern of the pattern roller and then fixedly apply powder of an active substance to the metallic porous foil.
The present invention provides an electrode for a battery which is produced by the above-described methods.
The electrode for a battery may be produced by forming voids consisting of fine gaps between adjacent metal powders and pores having a required configuration and being larger than the voids on the metallic porous foil and filling the powder of the active substance into the voids consisting of the fine gaps and into the pores larger than the voids.
That is, to increase the amount of the active substance per area, in addition to voids consisting of fine gaps that are generated between adjacent metal powders, large pores may be formed on a metallic porous foil similar to a conventional punched porous metal plate, and powder of the active substance may be filled into the large pores in addition to the voids consisting of the fine gaps.
The electrode for a battery may be produced by forming burr-formed pores having burrs projecting from one surface or both surfaces of the metallic porous foil or/and concave and convex bent portions on one surface or both surfaces of the metallic porous foil. The burrs and/or the concave and convex bent portions hold the layer of the active substance.
By forming the burr-formed pores to hold the active substance applied to the metallic porous foil with the burrs or/and by forming the concave and convex bent portions on the metallic porous foil to increase an apparent thickness thereof, it is possible to increase the amount the active substance which is held by the burrs or/and the concave and convex portions.
Preferably, the metallic porous foil has a plurality of void-unformed lead portions formed at regular intervals. The layer of the powder of the active substance is not formed on a surface of the lead portion.
The electrode produced by passing the active substance powder-applied base material (porous metallic foil) between a pair of pressure rollers has a thickness of 0.05 mm-6.0 mm. The metallic porous foil consisting of metal powder and serving as the base material of the electrode has a thickness 10 xcexcm-500 xcexcm. The void content of a fine gap between the adjacent metal powders is 5-30%. The metallic porous foil has a tensile force of 1 kgf/20 mm-30 kgf/20 mm and an elongation of 0.6-30%. The rate of large pore area of the metallic porous foil is 20-60%.
Although the kind of powder of the active substance which fixes to the metallic porous foil corresponds to the kind of battery, the following active substances can be used: metals such as zinc, lead, iron, cadmium, aluminum, lithium, and the like; metal hydroxides such as nickel hydroxide, zinc hydroxide, aluminum hydroxide, iron hydroxide, and the like; lithium composite oxides such as cobalt oxide lithium, nickel oxide lithium, manganese oxide lithium, vanadium oxide lithium and the like; metal oxides such as manganese dioxide, lead dioxide, and the like; conducting polymer such as polyaniline, polyacetylene, hydrogen-storing alloy, carbon and the like. When the active substance is filled into the base material of the electrode for a battery, according to the conventional method, a conductive agent such as carbon powder and a binder (binding agent) are added to the active substance. On the other hand, according to the present invention, the active substance is filled into the base material of the electrode without adding the binder thereto, as described above. The metallic porous foil of the present invention has fine voids into which the powder of the active substance can be filled without a binder. The void having a three-dimensional structure holds the powder of the active substance at a high degree of strength and thus can hold it without dropping it from the metallic porous foil. Non-addition of the binder to the active substance enhances the electricity collection performance of the electrode dramatically.
The present invention provides a battery having the above-described electrode. The electrode is most favorably used in a nickel hydrogen battery. In addition, the electrode is used in a nickel cadmium battery, a lithium primary battery, a lithium secondary battery, an alkaline dry cell, a fuel cell, and a battery for a vehicle.